1. Field of the Invention
The present invention relates to a multi-beam light source unit able to scan light beams from plural light sources at the same time and to scan a surface along plural lines, an optical scanning device having the multi-beam light source unit, an image formation apparatus having the optical scanning device, such as a copier, a printer, a facsimile, a plotter or others, a light beam combining unit used in the multi-beam light source unit, an optical system, and an optical apparatus having the light beam combining unit. The light beam combining unit of the present invention is applicable to a combination element used in optical communication.
2. Description of the Related Art
In these years, along with an increase of printing speed of image formation apparatuses, it is desired to improve writing density. For this purpose, in an optical scanning device constituting the image formation apparatus, it is attempted to increase the speed of light deflection of a light deflector, namely to increase the rotational speed of a polygon mirror, so as to increase the scanning speed and scanning density.
When the rotational speed is increased, however, noise and heat become a problem. Hence, there is a limit to the rotational speed.
On the other hand, Japanese Laid-Open Patent Application No. 2003-121772 (hereinafter referred to as “reference 1”) discloses a technique to increase the scanning speed and scanning density, specifically, a multi-beam light source unit is used to scan plural light beams at the same time and to write plural lines at the same time.
In a multi-beam light source unit able to scan multiple light beams at the same time, for example, a light source able to generate multiple light beams can be used to replace a conventional single-beam light source in an optical scanning device. For example, a light source able to generate multiple light beams may be a package including a laser array having multiple light emitting spots.
However, when fabricating a laser array light source, along with an increase of the light emitting spots, such as, 4, 8, . . . , it becomes technically more and more difficult, and this makes the laser array light source very expensive.
Many methods are proposed to use plural single-beam light sources (one package has one light emitting spot) to realize the multi-beam light source unit.
The single-beam light sources are inexpensive, and can be mass produced. For example, the cost of four single-beam light sources is lower than a multi-beam light source unit having four light emitting spots.
In order to fabricate the multi-beam light source unit by using many single-beam light sources, it is proposed to use beam combining elements, and several types of beam combining elements are proposed.
For example, Japanese Laid-Open Patent Application No. 9-189873 (hereinafter referred to as “reference 2”) discloses a beam combining element in which two light beams are incident on a half mirror at 90 degrees, and the two light beams are combined at nearly the same direction.
Japanese Laid-Open Patent Application No. 9-230260 (hereinafter referred to as “reference 3”) discloses a beam combining element in which two light beams are incident on a polarized light beam splitter or a polarized beam half mirror at 90 degrees, and the two light beams are combined to be in nearly the same direction.
Japanese Laid-Open Patent Application No. 2001-013433 (hereinafter referred to as “reference 4”) discloses a beam combining element in which two light beams are incident on a polarized light beam combining prism made from films roughly parallel to each other and the two light beams are combined at nearly the same direction.
In the beam combining elements disclosed in reference 2 or reference 3, the two light beams are incident at 90 degrees. This is required by the characteristics of the half mirror or the polarized beam splitter; specifically, these elements require the incidence angle of the light beam on the elements to be 90 degrees.
Due to this, the light source and the optical system have to be laid out to meet this requirement of a 90 degree incidence angle. This limits the degree of freedom of layout, and may require the multi-beam light source unit to be large sized, and in turn, cause the optical scanning device to be large.
Therefore, it is desirable to fabricate a beam combining element enabling a high degree of layout freedom.
In reference 4, in the polarized light beam combining splitter, it is required that two light beams be incident while being parallel to each other, namely, at zero degrees relative to each other. Similarly, this also limits the degree of freedom of layout.
From the point of view of combination of polarized light beams, the polarized light beam splitter and the polarized light beam combining prism disclosed in reference 3 or reference 4 are formed by bonding triangular prisms with thin films, and this polarized beam splitting film splits the incident light beam.
This thin film exhibits high beam splitting performance when being used at specified wavelengths and specified incidence angles, but suffers from the problem in that the beam splitting performance degrades significantly much when the wavelengths and incidence angles deviate from the specified values.